"Radiotherapy" is the use of radiation in treating cancer or cancer related disorders/illnesses. Each therapy program has a radiation dosage defined by type and amount of radiation for each treatment session, frequency of treatment session and total of number of sessions. In addition, the radiation may be delivered internally or by an external beam. Each of these factors and hence the radiation treatment program may be different for different patients/cases. In addition, there are a variety of devices or machines for planning and delivering the radiation at the prescribed doses, for instance, a CT scanner, a conventional simulator, a Cobalt-60 teletherapy machine, a linear accelerator, an after-loader for radioactive sources, and the like.
In external beam radiation therapy treatment, highly directional fluxes of high energy photon and/or electronparticles are delivered by medical linear accelerators or Cobalt-60 machines into the patient's body, through a specifically collimated cross section, often referred to as the "field" with reference to the patient. Angle of entry and shape of the radiation beam from such a machine is as carefully planned on a patient by patient basis as the radiation dosage itself. To illustrate this point, a typical scenario in providing radiotherapy to a cancer patient is as follows.
After the decision for radiotherapy has been made, a subject patient attends a planning session with the radiation oncologist and a medical physicist. At the planning session, all the diagnostic data (CT scans, radiographs, MRI scans) are assembled for the purpose of defining the target to be treated and the surrounding critical normal tissues to be spared. The patient is then placed under a simulator which mimics in all aspects the radiotherapy machine for the treatment, except that the beam from the simulator is a diagnostic X-ray beam. Since the radiation dosage from this diagnostic beam is very low compared to the treatment beams, many treatment scenarios can be simulated to assess the best combination of beams to hit the target and spare the normal tissues. The internal structures of the patient for the corresponding field are assessed by taking a radiograph. External coverage is assessed by looking at an outline of the radiation beam on the patient's skin which is typically visualized by using a radiologically divergent light source collimated identically through the aperture of the real radiation beam. This collimated light projection is called the "light field".
Conventional simulators and treatment machines are equipped to provide the light field. The projection of the light field on the patient's skin simulates the geometry of the real treatment (radiation) beam. The simulation process allows the radiation oncologist to evaluate the patient's external anatomy relative to the radiation beam and to mark particular entry points, e.g., the field center, field corners, to be used as reference points later in the treatment sessions. That is, during the projection of the light field on the patient, the radiologist uses a pen (or ink) to mark indications of the field center and corners on the patient's skin. In order to reproduce the simulated treatment, the light field of the treatment machine is aligned to these markings/indications at the beginning of each treatment/dosage.
One disadvantage of the above conventional simulation process is the existence of large uncertainties in the correlation of diagnostic data (e.g., CT, MRI) taken elsewhere and applied to the treatment position which is usually different from the position that the patient was set up for the diagnostic data. Yet it is extremely valuable to obtain 3-dimensional patient information for planning purposes. An alternative process is to use a CT-scanner to provide 3-dimensional patient information but scan the patient in the treatment position. This is becoming the state-of-the-art "simulation" process. However, the disadvantage of the new approach using the CT-simulator is that it is not usually equipped to produce the light field for the purpose described above. In those machines which produce a light field, patient setup for multi-beam treatment is time consuming and cumbersome, since the alignment of the light field with the markings have to be confirmed for all the beams before the treatment can start and yet this can only be done one beam at a time involving the motions of heavy machinery. Moreover, since the light field can only show the entrance point on patient skin surface, but not the exit points, the matching of opposite beams, as often required by the treatment, can be difficult.
Thus there is a need for an efficient and accurate system to project and localize treatment beams when CT-scanners are used for simulation.